This invention relates to code-division multiple-access (CDMA) systems and, more particularly, to systems and methods for suppressing multiple-access interference (MAI) in CDMA systems. It also applies to systems with little or no spreading.
Recently, there has been significant interest in multicarrier and code-division multiple-access (CDMA) systems. Because efficient fast Fourier transform (FFT) techniques and high-speed digital signal processors are available, multicarrier systems are now being developed for high-data-rate communications. The convergence of multicarrier and CDMA systems is based on the application of spreading codes to orthogonal frequency division multiplexing (OFDM). The use of multicarrier code-division multiple access (MC-CDMA) was first proposed in [1] and [2], where spreading sequences are used in the frequency domain instead of the time domain. Since the appearance of [1] and [2], there has been significant work on improving the performance of these systems for different channel conditions. An overview and comparison of MC-CDMA techniques is given in [3].
In CDMA systems, multiple-access interference (MM) is one of the main sources of interference. Multiuser detection techniques can successfully reduce the effects of MAI, but they require substantial knowledge about the interfering signals and spreading sequences. This is especially true when the spreading sequences are aperiodic, because the optimal detector changes significantly from bit to bit [4]. Due to the complexity of multiuser detection algorithms, there has been research on excising interference using single-user detectors that exploit structural differences in the signals of different users. Techniques that have been successful in direct-sequence code-division multiple access (DS-CDMA) suppress MAI using limited knowledge about the interfering signals. Advantages provided by these techniques relative to multiuser detectors are lower complexity, lower requirements for system-wide knowledge, and possibilities for simple adaptive implementations. Since the symbol matched filter is the optimal receiver for the additive white Gaussian noise (AWGN) channel, the techniques for multiple-access interference suppression often focus on processing the output of the symbol matched filter. In [5], the authors over-sample the output of a symbol matched filter in the time domain and combine the samples to maximize several different performance metrics in a DS-CDMA system utilizing long pseudo-random spreading sequences.
Although interference suppression for DS-CDMA has been well studied, techniques for interference suppression in MC-CDMA have not been thoroughly explored. With increasing mobility of users and the desire to increase the processing gain in MC-CDMA systems, frequency shifts of users that result from Doppler or oscillator inaccuracies may soon become significant, and could even approach the subcarrier frequency spacing.